Revealing mistake: When Luke is being fed to the Rancor, in Jabba the Hutt's dungeon, there are black outlines around the beast's legs, from the composite's blue screen special effect. This was edited out in the special edition rerelease.
Suggested correction: This was not a "mistake". The outlines showing around the beast's leg were due to the limited CGI technology at the time the film was made. With advances in special effects in the following decades, the filmmakers were able to enhance the CGI quality in later releases.
But it is a mistake to show that the Rancor is not real, and the outlines show that. It's certainly not intentional.
Not sure what you mean that the rancor is not real. Of course it's not real. The issue is, at that time, it was not technically possible to show the beastie without the lines showing. I classify a mistake as something that was not intentional. In this case, it was, due to the limitations of CGI in the 1980s.
Revealing mistake: After the heroes get snared and surrounded by Ewoks on Endor, the Ewoks mistake C3PO for a god and start bowing. There is one Ewok which only has holes where the eyes and mouth are and you can see the eyes and mouth of the person inside the Ewok costume. (01:07:50)
Revealing mistake: The shots at the end of the movie with the emperor are obviously lit from the front. This exposed too much of the emperor's head inside the cowl so it was darkened to make him look more menacing. Look for the added shadow - it looks like someone took a black magic marker and coloured on the film. (This may be corrected on the re-release).
Revealing mistake: When Darth Vader lands on the forest moon of Endor, he exits the shuttle and his head passes through it. (01:19:30)
Revealing mistake: When Han is running from the explosion of the shield generator on Endor, you can see his reflection in the plexiglass that is used to protect the camera from the explosion. Since the reflection isn't of Han's back, the plexiglass must be in front of him. (01:49:55)
Revealing mistake: When Luke and Darth Vader are fighing in the emperor's throne room, they pass in front of a great window. The stars are blurred and they shake. (01:52:30)
Revealing mistake: (Original theatrical release) When the imperial officer yells "Freeze", he is transparent and you can see the reactor through him.
Revealing mistake: In many far away shots of the sail barge you can see it is on a dune rather than in mid air.
Revealing mistake: When Luke jumps from Jabba's prisoner's skiff, you can see a stick supporting the "floating" skiff he jumps over to. (00:32:15)
Revealing mistake: When Luke is in his X-Wing preparing to make the jump to Dagobah, you can see the red text in the TV-type viewer in the cockpit has been superimposed as it remains stationary relative to the frame even when the cockpit is moving around.
Revealing mistake: On Jabba's sail barge, Luke's hand is wounded before it is shot at.
Revealing mistake: During the speeder bike chase, the speeders never cast any shadows on the ground.
Revealing mistake: When 3PO and R2 are being assigned to their droidly duties in Jabba's palace, there is a shot of a poor droid having hot irons branding his feet. The irons are never closer than an inch from the droid's feet, and the steam that allegedly comes from the burning is obviously being shot out of holes in the droid's feet.
Revealing mistake: After Wicket tells the rebels where the secret entrance to the base is, there's a shot of spaceships flying in space. An X-Wing passes through the Millennium Falcon and another one through a Calamarian cruiser.
Revealing mistake: Right before Luke leaps from the speeder bike to the scout trooper's speeder, there's a shot showing the front of the speeders. Notice the area surrounding the speeders in this shot. During the rest of the speeder bike chase, the speeders pass through this area several times, even though they are not flying in circles. This can be seen, for example, from the sunlight hitting the ground and the tree on the left side with a bush in front of it. Also, one of the shots where this area is seen is the shot before the speeders pass two more scout troopers, but the background has been flipped.
Revealing mistake: When C3PO and R2D2 are picked up in the sand, there is no sand falling off them. Obviously, C3PO's legs and R2D2 where just placed behind some sand (and not in it) or they used blue screen in this shot.
Revealing mistake: Look carefully at the close-ups of C3PO's waste where the exposed wires are. You can tell it's fake wiring on top of a black suit which is particularly noticeable when the actor inside the costume as his stomach is moving when he has to breathe.
Revealing mistake: When Luke is fighting on a skiff, he kicks one of Jabba's men in the head, causing him to fall off. Luke's foot doesn't make any contact with the person. (00:33:48)
Revealing mistake: Widescreen version only: After Luke cuts off Vader's hand, the Emperor says, "Now, fulfill your destiny, and take your father's place at my side." In the next shot, Luke's lightsaber blade goes through the railing, without damaging it. (01:49:15)
Revealing mistake: During the lightsaber duel, Vader throws his at Luke. The catwalk behind Luke explodes, but if you watch closely there are two explosions: one some inches over where the lightsaber actually hits and one far on the left.
